Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 231400 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1000 AM EDT Fri Jun 23 2017 .SYNOPSIS... Mild, humid air builds across the region today into tonight out ahead of the remnants of Cindy which will bring the threat of heavy rain and possible flooding Saturday morning. A cold front will sweep the remnants of Cindy out to sea through late Saturday, improving overnight. Seasonably warm and less humid conditions Sunday will be followed by cooler and somewhat unsettled weather for early next week. Warmer conditions return for the latter half of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1000 AM Update... Difficult forecast is on tap for today as guidance continues to poorly handle current conditions. Isolated showers that were across western MA moved northeastward towards the NH border. These showers are in association with the theta-e gradient and the higher moisture transport. Thus this will push SNE into a very moist airmass for the remainder of the day. Latest Satellite imagery shows dry slot working its way across CT and RI and towards MA. This is in association with dry air in the mid-levels and lower K values. This break in the clouds will help increase surface temps rather quickly into the 80s over the next few hours. Biggest question, if these breaks of clouds will be enough to trigger iso to sct showers and thunderstorms. See a few popping up across upstate NY but lapse rates are quiet poor over the region. Will continue to watch, but believe a trying trend will be in favor for the rest of the morning. Attention then turns to showers across the Mid-Atlantic/south of Long Islands associated with weak wave. Will these continue to develop and become more widespread? All the hi-res guidance as well as the HRRR shows that this bulk of showers will begin to fizzle thanks to very weak ridging aloft. Still expect a few hit or miss from this system, but not as widespread as what is currently occurring. Any strong storm that does develop could result in some gusty winds as LLJ is increasing aloft. However the main threat will be heavy rain as PWATS are currently 1.7 inches with more moisture on the way thanks to Cindy. Will continue to monitor for any flood related products later today. Previous discussion... Showers and thunderstorms for the early half of the day, potential for heavy rain. Consensus of forecast guidance has an initial wave of tropical energy associated with Cindy sweeping the region during the early half of the day. Collocated low- level convergence of high theta-E air per nose of H925-85 jet beneath a pseudo H925-7 warm frontal boundary with parent mid- level ascent. Especially if there is partial clearing in the morning presenting the opportunity for the boundary layer to destabilize, can see an opportunity where the present wave out of the Mid-Atlantic to maintain longevity, perhaps becoming stronger. Thinking mainly across the interior N/W away from S flow off the cooler ocean waters which would keep the surface more capped. However forecast guidance not so robust. Increasing moisture with precipitable waters rising towards 2 inches, 0-6 km bulk shear around 30 kts out of the SW, and CAPE around 1-2k j/kg. Am going to lean with high chance PoPs N/W across the interior. Think the high- res 3 km NAM and both WRF have the right idea. S/E becoming socked in with low clouds, possible fog, the region cooler at the surface is capped. May see some showers, perhaps heavy, but greater concern is N/W. Looking at the late morning into early afternoon period. Not so confident on severe weather outcomes but agree with the marginal risk advertised by SPC. Believe the main threat to be heavy rain and frequent lightning, some localized nuisance flooding concerns, with gusty winds and hail secondary threats. The later half of the day looks to be dry. Behind the morning wave, a combination of subsidence and mid-level warming around H6-8 puts the squash on any additional wet-weather activity, stabilizing. If throughout the day places remain partially clear, some locations could see highs into the low 90s such as the Merrimack River Valley. Continued cool S/E under lower clouds, possible fog. Otherwise, main theme for today is warm and muggy. Dewpoints well into the 60s, some places near 70. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight into Saturday... Possibility of heavy rain, flooding associated with Cindy leftovers. An anomalous tropical airmass surging NE out ahead of a cold front / baroclinic zone. The tropical-moist plume converging with higher theta-E ahead of a mean long-wave trough axis at the mid-levels and beneath decent diffluence within the upper levels, there is support for deep layer forcing yielding efficient warm-rain processes with freezing level heights at least above 10 kft, in this case around 14 kft. However outcomes are challenging to nail down and determine as they can often be underestimated or misplaced by forecast guidance. In the last several forecast runs, model solutions have waffled with the remnants of Cindy. It makes for subsequent challenges as to the possibility of flash flooding, if any one area is under particular threat of seeing an excessive amount of rain in a short period of time. But with this forecast there is continued preference to the high-res 3 km NAM and WRF forecast model solutions. Suspect a possibility of maintenance of shower, thunderstorm activity out of Upstate NY and PA across S New England during the evening hours. Convergent low- level forcing of high theta-E air beneath continued energy streaming NE with the tropical-moist plume. Given the lack of diurnal heating, main concern is the potential for heavy rainers, nuisance flooding. Then around morning, crux of heavy rain is expected as the remnants of Cindy clash with a cool front dropping S, clearing out through the afternoon. The 3 km NAM and WRF-ARW fit the forecast thinking, that the broad tropical-moist plume though focused with remnants of Cindy would also converge along the low to mid level cool front that sweeps across the region through afternoon. Low confidence at this point concerning exactly where the greatest threat will be. Will continue to highlight possible flooding hazards in the HAZARDOUS WEATHER OUTLOOK. Can`t rule out potential for rain- fall rates of around an inch per hour and excessive rainfall amounts greater than 2 inches storm-total. Low confidence with respect to forecast grid rainfall amounts that add up around 0.50-0.75 inches given the difficulty in determining exactly where the heaviest rainfall will occur. The cool front sliding through late in the day, will see potential blustery SW winds depending on the ability to mix down, perhaps brought down mechanically via heavy rain given a saturated profile that is moist adiabatic. Prior to and throughout, low clouds and fog will likely remain an issue for the S-coast with the possibility of visibility down to a quarter mile or less requiring headlines. Will monitor for now. Behind the front winds back W and we begin to see partial clearing. Temperatures difficult to forecast given some uncertainty on the timing of the front and how quickly we clear out. Could warm into the mid 80s but will keep it conservative with seasonable highs around the upper 70s to low 80s. With the warm up may see some brief breezy westerlies behind the front with boundary layer mixing. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Seasonably warm and less humid on Sunday * Cooler and somewhat unsettled pattern for early next week with showers possible at times * Warmer weather returns toward the end of next week Overview... Longwave trof over the Gt Lakes region will gradually slide east to New Eng Tue/Wed timeframe as multiple shortwaves rotate through the flow. This will bring a somewhat unsettled pattern for the first half of the week with cooler temps, near or a bit below normal. Models signaling a pattern change for the end of the week as ridging builds northward along the east coast which should bring warmer conditions for the end of next week. Details... Saturday night and Sunday... Cold front expected to be moving off the coast Sat evening followed by drier conditions during the night. Low risk of a lingering evening shower for the Cape/Islands, otherwise clearing skies during the night with cooler and much less humid airmass moving in. The dry airmass will continue into Sunday with sunshine mixing with diurnal cu in the afternoon. Fairly robust mid level shortwave will move NE from the Gt Lakes with a secondary cold front moving into SNE. Brunt of shortwave energy passes to the north and west but can`t rule out a few showers developing in the interior in the afternoon. 850 mb temps 12-13C and soundings indicate deep mixing on Sunday above 850 mb so temps may overachieve. Expect highs reaching low to mid 80s, except a bit cooler along the south coast with SW flow. Comfortable humidity levels with dewpoints in the 50s. Monday into Wednesday... Mid level trof gradually moves into New Eng as series of shortwaves rotate through the flow. Cooling temps aloft with 500 mb temps reaching -18 to -20C will likely bring some diurnal showers at times although certainly not a washout and expect a mix of clouds and sun. Any showers Mon should be confined to the interior, but all areas Tue as the trof moves into the region. Best chance for any thunder will likely be Tue with TT into the lower 50s and 500 mb temps approaching -20C. Upper trof begins to slide to the east on Wed. Cold pool aloft lingers but moisture is limited so expect limited coverage of any showers Wed. Temps slightly below normal with highs mainly in the 70s and lows in the 50s. Thursday... As mid level trof exits, rising heights into New Eng as SE US ridge builds north. High pres moves off Mid Atlc coast with good warm advection. Warmer conditions expected with temps possibly reaching well into the 80s in some locations. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...Moderate confidence. Today... IFR/LIFR stratus and fog along the S-coast with SCT-BKN low- end VFR elsewhere. Brief improvement possible at times along the S-coast up to VFR but IFR may linger over the islands. S winds around 10 to 15 kts with gusts up to 25 kts, strongest across E/SE New England. Initial showers moving through western New Eng this morning with more SHRA/TSRA chances expected over N/W CT and N/W MA, mainly afternoon into early evening. TEMPO MVFR/IFR with RA/+RA. Tonight into Saturday... MVFR-LIFR CIGs with MVFR-IFR VSBYs with BR/FG. SHRA/TSRA early tonight approaching from the W. RA/+RA approaching from the SW towards morning, exiting around midday ahead of a wind shift from the W. Prior to the wind shift will continue to see SW winds with potential gusts 25 to 30 kts LLWS possible for SE coastal terminals as is some embedded TSRA. Improving towards evening to low-end VFR / SKC. KBOS Terminal...VFR today with gusty SW winds up to 25 kts, lower CIGs into this evening with increasing rain chances. KBDL Terminal...May need to watch for possible SHRA/TSRA NW of the terminals around late morning into early afternoon. Otherwise low-end VFR CIGs today with blustery SW winds that will lower into evening prior to onset of RA/+RA. Outlook /Saturday Night through Tuesday/...High confidence. Sunday and Monday...Mainly VFR with sct-bkn diurnal clouds. A few diurnal showers possible in the interior Tuesday...Mainly VFR cigs with a few showers/isolated t-storms possible. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... 1000 AM update...Went ahead an issued a SCA for BOS harbor and Narragansett Bay. Winds gusts of 25 kts are possible during the later half of the day. SW winds on the increase with gusts up to 25 to 30 kts through early Saturday prior to a cool frontal passage. Waves building around 5 to 6 feet on the waters as the remnants of Cindy are advected NE across the waters around Saturday morning. Prior to and during this time, likely to be either a combination of low clouds and fog with visibility down to a quarter mile or less possible. Improving late Saturday as winds become westerly while beginning to diminish along with wave action. Outlook /Saturday Night through Tuesday/...High confidence. Sunday...Increasing SW flow but speeds should remain below SCA with a few gusts 20-22 kt nearshore waters in the afternoon. Monday and Tuesday...Quiet boating weather with winds and seas below SCA. && .TIDES/COASTAL FLOODING... Astronomical tides are rather high through this weekend during the night time cycles. Boston has a 12.3 ft tide just before midnight tonight, a 12.4 ft high tide around midnight Sat night and 12.2 ft just after midnight Sun night. Fortunately, offshore winds are forecast with minimal or no surge so do not anticipate any issues. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 1 PM this afternoon to 11 AM EDT Saturday for ANZ231>234. Small Craft Advisory from 1 PM this afternoon to 9 PM EDT this evening for ANZ230-236. Small Craft Advisory from 8 PM this evening to 8 PM EDT Saturday for ANZ235-237. Small Craft Advisory from 2 PM this afternoon to 11 AM EDT Saturday for ANZ250. Small Craft Advisory from 2 PM this afternoon to 8 PM EDT Saturday for ANZ254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...Dunten/Sipprell SHORT TERM...Sipprell LONG TERM...KJC AVIATION...KJC/Sipprell MARINE...KJC/Sipprell TIDES/COASTAL FLOODING...Staff is the U.S. government's official web portal to all federal, state and local government web resources and services.